The use of radiation-sensitive silver halide emulsions for medical diagnostic imaging can be traced to Roentgen's discovery of X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913 the Eastman Kodak Company introduced its first product specifically intended to be exposed to X-radiation.
The desirability of limiting patient exposure to high levels of X-radiation has been recognized from the inception of medical radiography. In 1918 the Eastman Kodak Company introduced the first medical radiographic product which was dual-coated--that is, coated with silver halide emulsion layers on the front and back of the support.
At the same time it was recognized that silver halide emulsions are more responsive to light than to X-radiation. The Patterson Screen Company in 1918 introduced matched intensifying screens for Kodak's first dual-coated (Duplitized.TM.) radiographic element. An intensifying screen contains a phosphor that absorbs X-radiation and emits radiation of a longer wavelength, usually in the near ultraviolet, blue or green portion of the spectrum.
While the necessity of limiting patient exposure to high levels of X-radiation was quickly appreciated, the question of patient exposure to even low levels of X-radiation emerged gradually. The separate development of soft tissue radiography, which requires much lower levels of X-radiation, can be illustrated by mammography. The first intensifying screen-radiographic film combination for mammography was introduced in the early 1970's. Mammographic film contains a single silver halide emulsion layer and is exposed by a single intensifying screen, usually interposed between the film and the source of X-radiation. Mammography employs low energy X-radiation--that is, radiation which is predominantly of an energy level less than 40 keV.
In mammography, as in many forms of soft tissue radiography, pathological features sought to be identified are often quite small and not much different in density than surrounding healthy tissue. Thus, relatively high average contrast, in the range of from 2.5 to 3.5, over a density range of from 0.25 to 2.0 is typical. Limiting X-radiation energy levels increases the absorption of the X-radiation by the intensifying screen and minimizes X-radiation exposure of the film, which can contribute to loss of image sharpness and contrast.
As radiologists began to generate large volumes of medical diagnostic images, the need arose for more rapid processing. The emergence of rapid access processing is illustrated by Barnes et al U.S. Pat. No. 3,545,971. Successful rapid access processing requires limiting the drying load--that is, the water ingested by the hydrophilic colloid layers, including the silver halide emulsion layers, that must be evaporated to produce a dry image bearing element. One possible approach is to foreharden the film fully, thereby reducing swelling (water ingestion) during processing. Because silver image covering power (maximum density divided by the silver coating coverage) of silver halide medical diagnostic films was markedly reduced by forehardening of the films, it was for many years the accepted practice not to foreharden the films fully, but to complete hardening of diagnostic films during rapid access processing by incorporating a pre-hardener, typically glutaraldehyde, in the developer. Dickerson U.S. Pat. No. 4,414,304 (hereinafter referred to as Dickerson I) demonstrates full forehardening with low losses in covering power to be achievable with thin (&lt;0.3 .mu.m) tabular grain emulsions.
Since adopting full forehardening of tabular grain silver halide emulsion containing radiographic elements further efforts to reduce the drying load placed on the rapid access processors has largely focused on limiting the hydrophilic colloid content of the medical diagnostic elements. However, when the hydrophilic colloid content of the emulsion layer falls too low, the problem of wet pressure sensitivity is encountered. Wet pressure sensitivity is the appearance of graininess produced by applying pressure to the wet emulsion during development. In rapid access processing the film passes over guide rolls, which are capable of applying sufficient pressure to the wet emulsion during development to reveal any wet pressure sensitivity, particularly if any of the guide rolls are in less than optimum adjustment.
Since mammographic films locate all of the silver halide emulsion on one side of the support, the resulting layer unit contains higher silver halide and hydrophilic colloid coating coverages and hence larger amounts of water ingested during development and fixing that must be removed during drying than the layer units of a dual-coated film, which approximately halves the silver halide and hydrophilic colloid per side by dividing the silver halide and hydrophilic colloid equally between front and back layer units. Thus, conventional dual-coated films are capable of more acceleration of rapid access processing than mammographic films.
There are several problems that have kept mammographic films from successfully adopting dual-coated formats and thereby improving their rapid access processing capability. Dual-coated films have been conventionally exposed with a front and back pair of intensifying screens. The front screen is provided to expose the layer unit on the front side of the film support and the back screen is provided to expose the layer unit on the back side of the support. Unfortunately some of the light emitted by the front screen also exposes the back layer unit and some of the light emitted by the back screen exposes the front layer unit. This results in a reduction in sharpness and is referred to as crossover.
Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 (hereinafter collectively referred to as Abbott et al) demonstrate that spectrally sensitized tabular grain emulsions are capable of reducing crossover to less than 20 percent--that is, less than 20 percent of the light emitted by the front screen is transmitted to the back layer unit.
Subsequently, Dickerson et al U.S. Pat. Nos. 4,803,150 and 4,900,652 (hereinafter referred to as Dickerson et al I and II) demonstrated an arrangement for essentially eliminating crossover by employing spectrally sensitized tabular grain emulsions in combination with front and back coatings that contain a particulate processing solution decolorizable dye interposed between the front and back emulsion layers and the support. Unfortunately, this requires two additional hydrophilic colloid layers to accommodate the added processing solution decolorizable dye. Nevertheless, Dickerson et al I and II demonstrate management of hydrophilic colloid in a dual-coated format to realize the advantage of accelerated rapid access processing.
Luckey et al U.S. Pat. No. 4,710,637 represents an unsuccessful attempt to undertake mammographic imaging using dual-coated film. To allow a front and back pair of intensifying screens to be employed in combination with a dual-coated film, Luckey et al found it necessary to thin the front screen to limit its absorption of low energy X-radiation. Although the teachings of Luckey et al and Abbott et al and eventually those of Dickerson et al I and II were all employed, the commercial sale of dual-coated mammographic film was discontinued for lack of acceptance by radiologists. The radiologists found pathology diagnoses to be unduly complicated by objectionable image characteristics that could not be eliminated. Use of a front and back intensifying screen pair to expose the dual-coated film increased the sharpness (MTF) and X-radiation transmission requirements for the front screen as compared to a single screen, single emulsion layer unit imaging system, leading to unattainable uniformity requirements for the front screen phosphor layer. In other words, the dual-coated films failed to produce mammographic images acceptable to radiologists, since they placed performance requirements on the front screens of the intensifying screen pairs used for their exposure that could not be satisfied.
Typically dual-coated silver halide medical diagnostic films are processed in a rapid access processor in 90 seconds or less. For example, the Kodak X-OMAT M6A-N.TM. rapid access processor employs the following processing cycle:
______________________________________ Development 24 seconds at 35.degree. C. Fixing 20 seconds at 35.degree. C. Washing 20 seconds at 35.degree. C. Drying 20 seconds at 65.degree. C. ______________________________________
with up to 6 seconds being taken up in film transport between processing steps.
A typical developer (hereinafter referred to as Developer A) exhibits the following composition:
______________________________________ Hydroquinone 30 g Phenidone .TM. 1.5 g KOH 21 g NaHCO.sub.3 7.5 g K.sub.2 SO.sub.3 44.2 g Na.sub.2 S.sub.2 O.sub.3 12.6 g NaBr 35.0 g 5-Methylbenzotriazole 0.06 g Glutaraldehyde 4.9 g Water to 1 liter/pH 10.0 A typical fixer exhibits the following composition: Sodium thiosulfate, 60% 260.0 g Sodium bisufite 180.0 g Boric acid 25.0 g Acetic acid 10.0 g Water to 1 liter/pH 3.9-4.5 ______________________________________
Radiation-sensitive silver halide containing radiographic film for recording medical diagnostic images of soft tissue (e.g., mammographic film) through exposure by a single intensifying screen located to receive X-radiation and emit light to the film have required all of the latent image-forming silver halide grains to be coated on one side of the support to achieve optimum levels of image sharpness. This in turn requires a higher coating coverage of hydrophilic colloid than is employed on either side of dual-coated radiographic films. The higher hydrophilic colloid coverages limit the extent to which rapid access processing can be accelerated. Thus, currently mammographic and similar soft tissue imaging medical diagnostic films are coated in a single-sided format to maximize image sharpness and uniformity, but cannot achieve the higher rates of rapid access processing finding increasing use in processing dual-coated radiographic films.
An attempt by Luckey et al, cited above, to provide mammographic film in dual-coated format was ultimately rejected by radiologists for failing to provide images of acceptably high sharpness and low mottle.
No medical diagnostic radiographic film for imaging soft tissue, such as mammographic film, has heretofore been available combining high levels of image sharpness and uniformity and the capability of accelerated rates of rapid access processing attainable with dual-coated radiographic films.
Dickerson et al U.S. Pat. No. 5,738,981 discloses the use of a dual-coated film exposed from one side by a cathode ray tube, photodiode or laser for reproducing digitally stored medical diagnostic images through exposure and processing, including development, fixing and drying, in 90 seconds. Since this film is intended to be used only to reproduce images that have already been captured by X-radiation exposure and converted to a digital form, the construction of the film is chosen to minimize image noise and to maximize processing convenience at the expense of radiation sensitivity. Thus, design considerations are quite different from that of medical diagnostic imaging that exposes a patient to X-radiation. Tabular grain emulsions are not preferred. Instead preferred emulsions have mean grain ECD's of less than 0.4 .mu.m. Additionally high chloride emulsions are preferred to facilitate processing, even though high chloride emulsions generally exhibit lower sensitivity than high bromide emulsions with otherwise comparable grains.